Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: t!mortician on November 17, 2020, 05:37:33 PM
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So I have a 1986 chevy short bed with the eddelbrock preformer rpm top end kit on a 78 454 bored .30 over 9.5 to 1 compression cam specs are Duration Advertised 335° Intake/329° Exhaust
Duration @ .050'' 236° Intake/245° Exhaust
Lift @ Valve .625'' Intake/.639'' Exhaust
Lift @ Cam .368'' Intake/.376'' Exhaust
Lobe Separation Angle 112°
Intake Centerline 109°
Intake Timing @ .050" Open 6° BTDC
Close 51° ABDC
Exhaust Timing @ .050" Open 54° BBDC
Close 11° ATDC
Also I have 9 to 11 lbs of vacuum at idle
Carb is 17080212. I am thinking recipe # 2 out of the book but kinda new. If there is a thread on this already I can't find it so any help would be great
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Also headers and 3 inch Exhaust. I am going to order ax rods, .035 needle and seat, full taper primary rods to .026 tip, cam and spring set, and B secondary hanger. I will follow recipe 2 and post how it goes any comments or suggestions are greatly appreciated
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You will find that carb already pretty generous for idle fuel. We can discuss the specifics when you call in the parts order. I'm here 7am-4pm, M-F EST...tks......Cliff
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Ok thank you I will give you a call tomorrow 🙂
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So after talking with cliff I ordered a kit with 72 jets, his primary metering rods, a new choke pull off, and it had an orange or red spring for the power piston. I also opened idle tubes from. 036 to .038 and the idle down channel fromb.050 to .055. I put a set of ax secondary
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So I now have some control over the idle screws but i can't seem to get the power I want or expect at any point in acceleration the only thing I have noticed is that the power piston won't seat until about 2000 rpm. I think I have the lightest spring already any suggestions 🤔
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The power piston is controlled by engine vacuum, so it is a load sensing device just like the vacuum advance in the distributor.
It will always be applied or in the down position at idle and "normal" driving at light throttle/light load.
If you aren't making enough vacuum at idle or when you are driving the vehicle gently it's there is something wrong someplace unrelated to the carburetor.
Most well thought out engines, even those that make a lot of power will make at least 10-12" vacuum at idle speed and a LOT more than that at light part throttle cruising. They will nearly bury the vacuum gauge coasting.
Take some vacuum readings at idle, and during "normal" driving scenarios and post the results......
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Curious, What type of cam that is. Roller or flat tappet. seems like a big spread between advertised duration and @.050 specs.
Assuming its an automatic trans, do you know your vacuum at idle in gear?
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Edelbrock doesn't make camshafts, they source the from CMC just like Speed Pro, Sealed Power, FM and about a dozen other companies have for decades.
The cam is most likely rated at .002" tappet lift instead of .006" like most Cam Companies rate them, so you're going to see a much wider spread between advertised and .050" duration numbers.
What the engine needs here is about one more full point of compression to be happy.
I have a 455 engine in my Ventura with CNC ported aluminum heads and 236/245/112 LSA cam with the ICL at 109, lift at the valves is .639", so a very similar build.
It idles just about smooth with a very slight hint of "attitude" making 13.5" vacuum at 750rpm's.
The BIG difference is that my engine has 11.3 to 1 compression instead of 9.5". It would NOT idle well clear down at 9.5 to 1. It would also have poor throttle response and lackluster performance until I got it past 3500rpms or so.
What is really needed here is more compression for that much camshaft in this particular build.....IMHO.......
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Too many variables here. Street vehicle or track warrior? Pistons,Rear axle ratio, stall converter, tire size?
If this is a true street vehicle that cam would be better suited to more cubic inches. If those are stock flat top pistons it might not be making even 9.5:1 compression.
A 454 (true street engine) could make More power, better drivability, and use less fuel with less duration and tighter LSA camshaft.
Edelbrock makes excellent intake manifolds and cylinder heads. There camshaft choices not so good. Low intensity (too slow lifting the valve) and wide LSA to get the engine to idle better with more duration. This especially does not favour low compression.
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The worst cam you will ever put in a big CID engine with moderate compression will have short seat timing and tight LSA.
Sure they will pack some punch and wake things up by yanking the power down some, but you can very quickly run into detonation issues even at relatively "low" compression ratios.
Case in point below with an engine I got involved with:
The engine builder (owns a speed shop and dyno) called Comp for a cam, and they sent him an XR276HR for his 455 engine with 250cfm head flow and 9.3 to 1 compression. Sounds like a great choice, right? Not much more seat timing than a 350SBC 300hp cam, 224/230 @ .050", 110LSA, etc. It didn't make chit for power and pinged pretty hard on the dyno resulting in rod bearing replacement.
I was contacted as everyone at the dyno facility right down to the guy taking out the trash was blaming the Q-jet for "not keeping up with their engine".
Instead of messing with carbs I suggested a cam change with more seat timing, 114LSA and ICL at 110. The engine builder thought I was nuts!
He did it anyhow, no other changes and a couple weeks later called and told me that the engine was a completely different beast. It made nearly 90 MORE horsepower on the dyno, idled smoother, and no detonation whatsoever.
Once the engine reached the vehicle the owner reported that it was very "mild" at a glance, but excellent street manners and more power than any type of street radial could ever hold!.......Cliff
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those are some pretty vague statements. Not sure if you understand seat timing
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What's not to understand?
I helped a customer who couldn't make 425hp with a 455 CID engine at 9.3 to 1 compression because he trusted the tech and Comp to supply his cam and took him to nearly 520hp with more power (torque) at every RPM and it quit pinging on pump gas.
The engine builder and the owner also were remarkably surprised at how well the engine idled with the larger cam in it and absolutely love the end result once it made it into the vehicle......FWIW......Cliff
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Maybe there was more wrong with the initial build then the builder told you.
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No, they only made ONE change.
The engine builder actually owns a machine/speed shop, and dyno. He specializes in Big Block Chevy and Ford engines, but this was his first Pontiac build.
He bought a complete rotating assembly to stroke a 400 block to 467CID, zero decked, tight quench, ported the factory iron heads to 250cfm, etc.
The missing piece of the puzzle was the camshaft, picked it too small and LSA too tight. One has to realize that a 455 Pontiac is a poor design from the start, although it is capable of big power. The stroke is long, piston speed high, heavy internals and fed by cylinder heads with no more cross section in the intake runners than a decent small block Chevy head.
They are excellent "mid-range" engines if you know hot to exploit them, but using tiny cams on tight LSA's just makes them too good at what they already do best, which is making all the power right off idle.
I suppose the XR276HR cam may work OK in a 454 Chevy built with peanut port or PASS round ports on it, but it's not nearly enough cam for a 455 Pontiac build, or at least far from ideal as the dyno chart and results we got with a much larger cam on a wider LSA clearly show.......FWIW......
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Yes an under square engine compared to a 454 BBC. Different needs. Thanks for filling in the blanks.
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You are most welcome.
I'd also add that Pontiac cylinder heads used machined flat combustion chambers with 30 degree intake seats (with few exceptions) designed to enhance low lift and longer seat timing plus wide LSA. The cams for them were no more than .406" lift with the exception of the RAII/RAIV camshaft which was only .470".
As crappy as all that is they will run pretty good if you know how to exploit their best attributes........FWIW.....
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RAIV was .516" due to usage of 1.65:1 rockers for this engine.
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Interesting to hear how much power power was found by one part change. I know you had helped me with my 355sbc q-jet rebuild & tuning. Used your cam suggestion over my Extreme energy choice and made huge improvements in tuning, power, drivability and mileage.
Building a 396 and will be calling soon for rebuild kit and guidance on Q-jet for it. I'll run my cam choice by ya too for a educated opinion. :)
Have a Happy (and better than last) New Year.
Phil
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LOTS of opinions when it comes to camshafts.
I'm going to be one of the very few who has actually rented the dyno time and back to back tested multiple cams to see how these things really work.
I've also advanced and retarded cams then street and track tested after each movement.
Plus I tune for a living and get to see all sorts of troubled engine combinations brought here that need help.
As mentioned earlier the WORST cam you will ever put in a big CID engine is short seat timing and tight LSA. I could write a couple hundred pages here with details and specifics related to that topic, but it sort of goes against the grain and too many folks who are disciples of some of the famous "guru's" in this hobby get their panties all wadded up about it.
Just like the dyno sheet I put up earlier here. We made ONLY a cam change, the engine was on the dyno and pinged hard enough with the small cam on a tight LSA to require rod bearing replacement. With the larger cam it idled better, improved throttle response, and WAY more power across a much broader RPM range.
Sadly this sort of information just goes on deaf ears as folks get pretty set in their ways and refuse to see the forest for the trees.
What also adds to the confusing with these things is miss-information on the Boards. The biggest myth in the hobby these days is LOWERING compression to 9.5 (or less) for pump gas. Not sure who came up with that nonsense but for sure stuffing a small cam in there on a really tight LSA with early intake closing is NOT going to bring back all the lost power nor will it necessarily make that engine pump gas friendly.
I custom tune on weekends, have now for nearly 20 years. I get a LOT of those engines brought here where the customer, all his friends, beer drinking buddies, local shops and all the "guru's" in his area have taken a stab at fixing it with no success.
To date the record for longest distance a vehicle has been brought here to custom tune and correct running issues with is 1800 miles one way! Most come in from 200 miles or less. Believe it or not the customer who brought his vehicle here 1800 miles needed a $30 part to fix his issues! I also went back thru everything in his carb and distributor as too many hands had been in that stuff, but the MAIN culprit as a simple part that took less than 2 minutes to install!
Remarkably most vehicles brought here to tune are fixed by going into the distributor before I even touch the carburetor. I've come to LOVE these cheap POS advance weight/spring kits. Absolute and total JUNK and cause WAY more running issues and poor end results with these engines than any other reason. 99 percent of the HEI "clones" and aftermarket "bug zapping" distributors aren't making the grade either and I've seen enough of these points eliminating modules throwing a monkey wrench into the deal to avoid them completely, but all that stuff is stories for another day........Cliff